Generally, a conventional automatic transmission for a vehicle includes a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction members actuated by hydraulic pressure for selecting a gear range of the gear shift mechanism.
A hydraulic control system for automatic transmissions operates by the selection of friction members using hydraulic pressure, created by the pumping of oil by a hydraulic pump, passing through a control valve. As a result, shifting the driving state of the vehicle can be realized automatically and appropriately.
Referring to FIG. 7, the above hydraulic control system includes a torque converter 2 for transmitting torque from an engine to a gear train, and an oil pump 4 for pumping oil needed to form hydraulic pressure for controlling the torque converter 2 and each speed ratio, and to lubricate the system.
The following are interconnected to form a pressure regulator and a damper clutch controller: a pressure regulating valve 8 which regulates the pressure of the oil supplied by the oil pump 4 and flowing through a line 6, a torque converter control valve 10 which controls the pressure of oil used for the torque converter 2 and for lubrication, and a damper clutch control valve 12 for increasing power transmission efficiency of the torque converter 2.
Also, lines are formed to supply part of the oil pressure generated in the oil pump 4 to a reducing valve 14, which reduces the level of pressure so that it is lower than line pressure, and to a manual valve 16, which operates according the positioning of a select lever (not shown) by a driver and converts the supply of pressure to different lines.
Lines are formed such that the hydraulic pressure, reduced to a fixed level of pressure in the reducing valve 14, is able to be used as control pressure for a high-low pressure valve 18. The high-low pressure valve 18 reduces line pressure in a high speed range to minimize the loss of drive effectiveness of the oil pump 4, and a pressure controller is formed for supplying part of the above hydraulic pressure to a first pressure control valve 20 and a second pressure control valve 22 such that the pressure can be used as shift range control pressure.
Further, lines are formed for allowing part of the hydraulic pressure supplied to the first and second pressure control valves 20 and 22 to be used as control pressure for an N-R control valve 24', which reduces shift shock when shifting from the neutral N range to the reverse R range.
A shift control valve 28, for converting pressure lines by the operation of first and second solenoid valves S1 and S2, communicates with a line 26. Hydraulic pressure flows through the line 26 when the manual valve 16 is in a drive D range. The first and second solenoid valves S1 and S2 are ON/OFF controlled by a transmission control unit (TCU). The shift control valve 28 together with the manual valve 16 comprise is manual and automatic shift controllers
As discussed in more detail below, a first speed line 44 branches off from the line 26. A second speed line 30, a third speed line 32, and a fourth speed line 34 are connected to the shift control valve 28. Shift valves of a hydraulic pressure distributor, for controlling each shift range, are connected to each of these lines such that control pressure can be supplied.
Namely, the second speed line 30 is formed to allow the supply of hydraulic pressure to a left-end port of a 1-2 shift valve 36 to control the same, the third speed line 32 is formed enabling the supply of hydraulic pressure to a left-end port of a 2-3/4-3 shift valve 38 to control the same, and the fourth speed line 34 is formed to permit the supply of hydraulic pressure to a right-end port of the 2-3/4-3 shift valve 38 and a left-end port of a 2-4/3-4 shift valve 40 to control both valves.
The first pressure control valve 20 controls line pressure under the control of a third solenoid valve S3, while the second pressure control valve 22 controls line pressure under the control of a fourth solenoid valve S4.
The first speed line 44 supplies hydraulic pressure to the first and second pressure control valves 20 and 22 and the third and fourth solenoid valves S3 and S4 operate such that hydraulic pressure is supplied to a first friction member C1, which is an input member of a first speed stage, via the 2-4/3-4 shift valve 40.
A timing control line 45 is connected to the first speed line 44 to enable line pressure flowing through the first speed line 44 to be supplied to a shift timing control valve 42.
The shift timing control valve 42 performs the function of either supplying or releasing operational pressure to a third friction member C3. The third friction member C3 acts as an input member in the third and fourth speed stages. The shift timing control valve 42 also regulates the timing for supplying hydraulic pressure to the third friction member C3 and to regulate the control timing for a second friction member C2, which operates as a reaction member, in second and fourth speed stages.
The shift timing control valve 42 is able to switch the operational pressure of the second friction member C2, operating as a reaction member, with that of the third friction member C3 when a fifth solenoid valve S5 is controlled to OFF by the TCU.
Also, a line 270 is formed to supply hydraulic pressure to the second friction member C2, operating as a reaction member of the second speed stage, from the first pressure control valve 20 via the 1-2 shift valve 36.
Further, a line 272 is formed to supply part of the hydraulic pressure flowing through the 1-2 shift valve 36 to the third friction member C3, acting as an input member in the third speed stage, via the 2-3/4-3 shift valve 38 and the shift timing control valve 42. Another line 274 is formed to supply the hydraulic pressure supplied to the third friction member C3 to a release side chamber h1 of the second friction member C2.
There is further provided a line 276 which, when the manual valve 16 is in the reverse range, supplies the hydraulic pressure supplied to a reverse first control line 46. The reverse first control line 46 supplies the hydraulic pressure to a fourth friction member C4, operating as a reaction member in the reverse R range, via the 1-2 shift valve 36. A line 278 also connects a reverse second control line 48 with the manual valve 16 to operate a fifth friction member C5, which operates as an input member.
A check valve 50 is mounted in the reverse second control line 48, and delays the release of hydraulic pressure when operational pressure is released to improve shift quality.
A kickdown switch 52 is fixed to an operational side chamber h2 of the second friction member C2. The kickdown switch 52 turns OFF when hydraulic pressure is supplied to the operational side chamber h2, turns ON when hydraulic pressure is supplied to the release side chamber h1 of the second friction member C2, and sends a signal of its state in both cases to the TCU.
So that operational pressure of the second friction member C2 can be converted according to the shift range and speed stage so as to enable skip shifting, the 2-4/3-4 shift valve 40 has ports to enable the directing of hydraulic pressure from the timing control valve 42 to the second friction member C2.
The reference S6, appearing in the drawing but not yet described, refers to a sixth solenoid valve which controls the damper clutch control valve 12 to either operate or discontinue the operation of the same.
In the above structure, hydraulic pressure is supplied to the first friction member C1 in the forward first speed stage; to the release side chamber h1 of the second friction member C2 and the operational side chamber h2 of the second friction member C2 in the forward second speed stage; to the first friction member C1, the operational side chamber h2 of the second friction member C2, and the third friction member C3 in the forward third speed stage; to the third friction member C3 and the operational side chamber h2 of the second friction member C2 in the forward fourth speed stage; and to the fourth and fifth friction members C4 and C5 in the reverse R range.
In the above hydraulic control system, the N-R control valve 24' is able to control the pressure operating on the fourth friction member C4, which is a low-reverse brake, to prevent shift shock when shifting the select lever from the neutral N range to the reverse R range, and from the parking P range to the reverse R range.
The N-R control valve 24' of the prior art, as shown in FIG. 5, includes a first port 100 for receiving hydraulic pressure from the manual valve 16, a second port 102 for receiving hydraulic pressure that has been reduced in the reducing valve 14, and a third port 104 communicating with the fourth friction member C4.
A valve spool 106 is provided in the N-R control valve 24'. The valve spool 106 includes a first land 108 for selectively allowing fluid communication between the first and third ports 100 and 104, and a second land 110 receiving the hydraulic pressure supplied to the second port 102. An elastic member 112 is mounted to the left (in the drawing) of the first land 108 such that force is exerted against the valve spool 106 in a rightward direction (in the drawing).
Through the above structure, the N-R control valve 24' is operated by the duty control of the third solenoid valve S3, and generates reduced control pressure by the third solenoid valve S3 releasing pressure.
Accordingly, oil is irregularly exhausted, as shown in FIG. 6, through an exhaust port Ex of the third port 104. This leads to the unstable control of hydraulic pressure such that shift shock occurs when shifting from the neutral N range to the reverse R range and from the parking P range to the reverse R range.